1. Field of the Invention
This invention relates to secondary batteries employing microporous separators and having as the electrolyte an aqueous metal bromide solution containing a complexing constituent capable of forming a water immiscible complex with cathodic bromine.
2. Prior Art
As is well known in the art, electrochemical cells have been proposed which have one electrode with a high positive oxidizing potential and another electrode with a strong negative or reducing potential. Typical of such cells is the metal halogen cell in which the anode material typically is zinc or cadmium and the cathodic halogen typically is bromine. Among the advantages of such metal halogen cells is their extremely high theoretical energy density. For example, a zinc bromine cell has a theoretical energy density of 200 Wh/lb., i.e., watt hours per pound, and an electric potential of about 1.8 volts per cell.
Electrochemical cells of the foregoing type are known to suffer from a number of disadvantages. Most of these disadvantages are associated with side reactions which may occur in such cells. For example, during the charging process free bromine is generated in the cell. This free bromine is available for electrochemical reaction with the zinc anode thereby resulting in auto discharge of the cell.
In U.S. Pat. No. 4,105,829 there is disclosed a metal halogen cell which employs a circulating electrolyte system containing complexing agent to effectively remove cathodic halogen from the electrolyte during charging of the cell. Basically, the complexing constituent or complexing agent is one which in the presence of halogen forms a water immiscible halogen complex. This complex is separated and stored external the cell during charging and is returned to the cell during discharge.
Another typical feature of the metal halogen cell disclosed in the aforementioned patent is that a microporous separator is employed. Among other things, the microporous separator serves to prevent contact of the metal anode with the counterelectrode in the cell, and it reduces contact of the metal anode with cathodic halogen during charging of the cell.
Despite the significant improvement that is achieved with the aqueous zinc bromine battery disclosed in the aforementioned patent, coulombic inefficiencies still result in operating such cells since the amount of energy recovered from the cell is less than that which is put in during charging of the cell.